The use of certain additives for the purpose of reducing the flammability of various thermoplastic polymers is well known to those skilled in the art. Flame retardant thermoplastic compositions are desired for many fields of use, such as in home construction, automobile and aircraft manufacture, packaging, electrical equipment, and the like.
Many thermoplastics ignite at relatively low temperatures and others have high ignition temperatures. Among the former can be mentioned polystyrene, polyethylene, cellulose esters, e.g., cellulose acetate and cellulose nitrate, and polyesters, e.g., poly(ethylene terephthalate). These resins are generally regarded to be highly flammable. On the other hand, some thermoplastics, such as the aromatic polycarbonates, polyphenylene ethers, polysulfones, polybenzimidazoles, polyamides, and the like ignite at temperatures far in excess of the others, e.g., at above 700.degree. F., or so. Although not considered to be highly flammable, they still are potentially dangerous unless rendered flame retardant. For convenience, all such resins will be classified herein as "normally flammable" because, although there is a high ignition temperature, a polycarbonate, for example, can support combustion beyond 20 seconds after ignition and subsequent removal of the igniting source and, in particular, burning is seen in thin sections.
It is therefore desirable to provide compositions which will not support combustion beyond a few seconds, both for the highly flammable resins, but in particular, even for those of high ignition temperature.
In the art of rendering thermoplastic resins flame retardant many materials have been used as flame retardant additives. Many of the additives are halogen containing compounds and these have been fairly successful in accomplishing the desired result, but at some sacrifice in physical properties in parts molded from the compositions. Often, however, increasing the amounts of conventional halogenated flame retardant compounds has an undesirable over-plasticizing effect on the normally rigid molded parts. In addition, especially with the high ignition temperature thermoplastics, a point is reached above which increased amounts of the known flame-retardant compounds do not improve the flame-out property of the composition. Finally, most of the conventional halogenated flame retardant compounds, in addition to lessening the desirable properties of the base polymer, are somewhat volatile and "plate out" on the surface of the molded pieces. This problem is especially aggravated in the case of polyesters and the high ignition point thermoplastics such as polyphenylene ether resins, alone, or in further combination with high impact rubber-modified polystyrene resins (HIPS), which ordinarily are processed at temperatures above about 200.degree. C., at which point the conventional halogenated materials tend to plate out.
As representative of the present state of the art, Fox, U.S. Pat. No. 3,855,277, discloses flame retardant agents comprising low molecular weight polymers of a carbonate of a halogenated dihydric phenol alone or in combination with an inorganic or organic antimony-containing compound.
Sandler, U.S. Pat. No. 4,098,704, discloses that poly(alkylene oxides) end-capped with tetrahalophthalate ester or amide groups are useful to provide textiles with a flame-retardant finish. In one example, tetrabromophthalic anhydride is reacted with a diprimary amine end-capped polypropylene oxide, and a half acid-amide end-capped product is produced. However, the patent provides no teaching that any of the compounds will function efficiently as non-migratory internal flame retardants in thermoplastics.
Newkirk et al., U.S. Pat. No. 4,294,944 and 4,312,966, disclose that the reaction products of polyoxyalkylene compounds and halogenated aromatic diacids and anhydrides function as flame-retardant antistatic additives for polymer fibers. However, this patent makes it desirable to use the additives in the form of a coating on the surface of the fiber to provide a softening effect, or to take advantage of their migration to the surface to provide anti-static effects, and this would not be desirable in three dimensional molded articles, as mentioned above.
Sandler, U.S. Pat. No. 4,397,977, discloses a flame retardant composition comprising a tetrahalophthalate plasticizer and a halogenated resin, in which the plasticizer appears to have been made by reacting a tetrahalophthalate with a polyoxyalkylene compound or amino derivative thereof. In any event, the compounds are di-esters or half-acid esters, half-acid amides or half-ester amides. There is no disclosure of phthalimido end-capped units and no teaching that the plasticizers would be non-migratory.
Halogenated flame retardant agents are also disclosed in Haaf et al., U.S. Pat. No. 4,433,088, and Abolins, U.S. Pat. No. 4,456,720. Haaf et al. disclose compositions using an aromatic phosphate compound such as isopropylated triphenyl phosphate and an adhesion promoting agent selected from a polyolefin glycol or a polyamide wherein the composition is essentially free of a polyolefin. The Abolins patent discloses that using halogenated organic compounds with boron containing salts and esters which are stable at 250.degree.-300.degree.C. improve the flame retardant properties of thermoplastic compositions.
Admixtures of triaryl phosphates and various thermoplastic resins are disclosed in Axelrod, U.S. Pat. No. 4,526,917, to render thermoplastic compositions flame retardant.
However, halogenated and/or phosphorus-containing flame retardant agents used in the aforementioned references are still not entirely satisfactory, especially when incorporated into thermoplastic compositions, particularly thermoplastic compositions composed of polyphenylene ethers and high impact polystyrene. More specifically, triaryl phosphate based flame retardant agents may sometimes adversely affect the environmental stress crack resistance of thermoplastic resins, since they also form a liquid deposit on injection molds. Efforts to reduce the volatility of the phosphate containing flame retardants by increasing molecular weight have not always been successful because flame retardancy tends to be decreased.
With respect to adding halogenated materials such as brominated polycarbonates and brominated esters, as well as brominated polystyrene to normally flammable thermoplastic compositions to render the same flame retardant, such flame retardants have the drawback of being generally incompatible with the resins, especially polyphenylene ether resins, and they thereby adversely affect the physical and thermal resistance characteristics of the blended composition.
It has now been discovered that low molecular weight polyalkylene ether polymers and derivatives thereof can be provided with tetrabromophthalimide end groups, e.g., by reacting amino-terminated such polymers with tetrabromophthalic anhydride, and that these are very useful additives to render polymers flame retardant. Furthermore, the softening points of these low molecular weight products can be tailored to suit a particular resin system into which they are to be incorporated.
Such products can be used alone to render normally flammable thermoplastic compositions flame retardant. They may also be used in combination with synergists, such as phosphorous compounds, but particularly with inorganic or organic antimony compounds.
By way of illustration, a low molecular weight tetrabromophthalimide end-capped poly(ethylene oxide) or poly(propylene oxide) or a derivative thereof having a molecular weight of between about 400 and about 2000 and a bromine content of between about 5 and 55 percent by weight, and melting at less than 200.degree. C., is uniquely advantageous in rendering a polyphenylene ether-styrene resin composition flame retardant. Other polymers rendered flame retardant include poly(ethylene terephthalate), poly(butylene terephthalate), poly(bisphenol A carbonate), and an acrylonitrilebutadienestyrene terpolymer.
Such compositions, which may also contain antimony compounds, are "self-extinguishing" and meet the stringent requirements of the Underwriters Laboratories Bulletin 94 flame test, without loss of ultimate physical properties and without plate-out of the additive or volatilization during injection molding.
The additives also possess a significant plasticization effect, with very little tendency to over-plasticize, and this overcomes a major drawback in the present state of the art. Flame retardancy is as good or better than with triaryl phosphates without volatility problems, and as good as that of the brominated additives, but without incompatibility problems often observed.
It is a principal object of this invention, therefore, to provide a class of highly efficient, novel, low molecular weight polybrominated phthalimide end-capped poly(alkylene oxides) for use as flame retardant agents.
Another object of the invention is to provide improved flame retardant polymer compositions containing such low molecular weight flame retardant additives.
Yet a further object of the invention is to provide flame retardant compositions from which the additive does not volatize during, or remain on the surface after, molding.
Still another object of the invention is to provide plasticized, flame retardant molding compositions.
Other objects and advantages of the invention will become apparent from the following detailed description.